1. Field of the Invention
The present invention is directed to a magnetic resonance imaging tomography apparatus, and in particular to a circularly polarizing RF antenna for such an apparatus.
2. Description of the Prior Art
Magnetic resonance imaging devices are known in the art for producing tomograms of an examination subject based on the principle of nuclear magnetic resonance, as well as for conducting diagnostics of anatomical joints and for portraying blood vessels. As is known, an image is constructed by computational or mensurational analysis of integral proton resonance signals from the three-dimensional spin density, or from the relaxation time distribution, of the examination subject. The examination subject, such as a human body, is introduced into a strong, uniform magnetic field, referred to as the fundamental or basic field, which aligns the nuclear spins in the examination subject. Gradient coils are also present which respectfully generate magnetic fields in three dimensions. A radio-frequency (RF) antenna excites the nuclear spins and transmits the resulting emitted magnetic resonance signal to a receiver. The RF antenna is generally connected to a transmitter and to a receiver via matching capacitances, and via a transmission/reception diplexer.
As is known, circularly polarizing antennas require relatively little transmission power when used in such devices, because they essentially generate only the field components effective for the nuclear magnetic resonance, for example, field components having a left rotational sense. For example, such an antenna may consist of two linearly polarizing antenna systems arranged orthogonally relative to each other, which are connected to a transmitter and to a receiver via a 90.degree. directional coupler. A transmission signal which is fed into the antenna is supplied directly to one of the antenna systems, and is phase shifted by 90.degree. and supplied to the other antenna system. This generates the rotating field which is effective for nuclear magnetic resonance tomography. In the reception mode, the antenna represents two signal sources phase-shifted by 90.degree., and also represents two uncorrelated noise sources. The 90.degree. directional coupler supplies the in-phase sum of the signals to the receiver. An improvement by at least a factor .sqroot.2 in the signal-to-noise ratio is obtained with such antennas, and only about half of the supplied power is required in comparison to linearly polarizing antennas, because a counter-rotating field component is not generated. This is described in the Journal of Magnetic Resonance, Vol. 54 (1983), at pages 324-327.
For obtaining a magnetic resonance image of the head, an antenna which tightly surrounds the head is generally employed. Circularly polarized head antennas are preferred, because they have an improved signal-to-noise ratio as well as a lower power consumption and better penetration characteristics. in order that the head of the human body can be more easily introduced into such antennas, efforts have been undertaken to make the antenna divisible. Such antennas can be employed for other purposes, for example, for obtaining an image of a movement-restricted, injured knee joint. In a known embodiment of a nuclear magnetic resonance tomography apparatus having a linearly polarizing antenna, the hollow-cylindrical coil system is divisible along a cylinder axis such that a lower, firmly mounted half antenna and an upper, removable half antenna are present. Energy feed ensues only to the lower half, whereas the upper half is inductively coupled to the lower half. In one embodiment, a plug-type connection between the two halves can also be provided, and thus both halves can be connected to a power supply. Such an antenna is described in European Patent Application 0 142 760.
In devising a circularly polarizing RF antenna which is divisible so that one part of the hollow-cylindrical coil system can be removed, or hinged away from, the remainder of the coil system, a problem results in that the conductors carrying the resonant current must be interrupted. In the assembled condition of the radio-frequency antenna, there must be a galvanic connection made via contacts, for example, plug-in contacts, or there must be capacitative connection over relatively large areas. These contacts must carry all of the RF resonant current, and thus have a direct influence on the unloaded quality of the RF antenna, as well as on the stability of its resonant frequency.
A known antenna for a magnetic resonance imaging apparatus for producing tomograms of parts of a human body consists of four H-shaped conductors arranged on a cylindrical surface such that the lateral legs of the conductors form one part of a ring, and the conductors are connected to each other by a stripline proceeding in axial direction of the cylinder. In this circularly polarizing RF antenna, one of the conductor structures is capacitatively coupled to the two neighboring conductor structures, and forms a current loop in combination with these two neighboring conductor structures. For capacitative coupling, a ring of electrically conductive material, for example copper, is provided at both ends of the hollow cylinder. In combination with the legs of the H-shaped conductor structures, these rings respectively form a divided capacitance, which acts as a coupling capacitance, as described in the Book of Abstracts, Society of Magnetic Resonance in Medicine, Third Annual Meeting, Aug. 13-17, 1984, New York).